Particles for non-magnetic undercoat layer of magnetic recording medium, method thereof and magnetic recording medium

ABSTRACT

The present invention provides particles for a non-magnetic undercoat layer of a magnetic recording medium, which comprises acicular hematite particles having an average major axial diameter of not more than 0.3 μm, a geometrical standard deviation in the major axial diameter of not more than 1.50 and a BET specific surface area of not less than 40 m 2 /g, and containing a total amount of sodium of not more than 50 ppm calculated as Na. The acicular hematite particles have an excellent dispersibility in a vehicle so that a non-magnetic undercoat layer containing the particles is excellent in surface smoothness and strength. A magnetic recording medium using the non-magnetic undercoat layer is excellent not only in electromagnetic performance, but in storage stability.

This application is a division of prior application Ser. No. 09/420,008filed Oct. 18, 1999, now U.S. Pat. No 6,299,973.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to acicular hematite particles suitablefor a non-magnetic undercoat layer of a magnetic recording medium, moreparticularly, to acicular hematite particles containing the total sodiumamount of not more than 50 ppm, method thereof and a magnetic recordingmedium using said acicular hematite particles.

2. Description of the Prior Art

With a recent development of miniaturized and lightweight video or audiomagnetic recording and reproducing apparatuses for long-time recording,magnetic recording media such as a magnetic tape and a magnetic diskhave been increasingly and strongly required to have a higherperformance, namely, a higher recording density, a higher outputcharacteristic and a lower noise level.

Especially, much higher recording density of audio or video rapes isalways desired and carrier signals tend to move to a shorter wavelengthrange.

Meanwhile, since an influence of self-demagnetization becomes prominentas the recording wavelength becomes shorter, it is necessary to reducean influence of self-demagnetization by thinning of a magnetic recordinglayer with a view to higher recording density. That is supported, forexample, on page 312 of “Development of Magnetic Materials andTechnology for High Dispersion of magnetic Powder” published by Sogo Gijutsu Center (1982), “ . . . the conditions for high-density recordingin a coated layer type are that high output characteristic and low levelof noise with respect to short wavelength signals are maintained. Tomeet such conditions, it is required to have large coercive force Hc andresidual magnetization Br . . . and to have a thinner thickness of thecoated film . . . ”.

In light of the foregoing situation, it is proposed and practiced toreduce self-demagnetization by decreasing the thickness of an uppermagnetic recording layer and to solve the problems such as adeterioration in surface smoothness and a deterioration inelectromagnetic performance by forming on a, non-magnetic base film atleast one non-magnetic undercoat layer which comprises dispersingnon-magnetic particles such as hematite particles in a binder (JapanesePatent Examined Publication (Kokoku) No. 6-93297, Japanese PatentNon-examined Publication (Kokai) Nos. 62-159338, 63-187413, 4-167225,4-325915, 5-73882, 5-182177, 3-347017, 6-60362, etc.)

Moreover, with a development of miniaturized and lightweight video oraudio magnetic recording and reproducing apparatuses for longtimerecording, surroundings in which a magnetic recording medium is used andstored become diversified and it is reacted to have storage stabilitynot only in ordinary conditions, but in high temperature and highhumidity conditions.

As a cause of lowering the electromagnetic performance, the storagestability of a magnetic recording medium and the dispersion stability ofa coating composition, a water-soluble alkali metal, in particular, awater-soluble sodium contained in the magnetic recording medium ispointed out.

The Japanese Patent Non-examined Publication (Kokai) No. 9-22524, forexample, on page 3, column 3, lines 14-20 discloses; “ . . . When a freefatty acid increases and water-soluble Na and Ca contained innon-magnetic particles abound, Na and Ca salts of the fatty acid tend todeposit to thus afford an adverse effect to the electromagneticperformance such as output performance and C/N, but by reducing thosesalts to the specific amount or less, an excellent storage stability anda low friction coefficient are obtained without a deterioration inelectromagnetic performance. The Japanese Patent Non-examinedPublication (Kokai) No. 62-209806 discloses on page 2, left uppercolumn, lines 3-19;” . . . the residual Na⁺ has been known to have agreat influence on the quality of a magnetic coated film. As a typicalexample, a so-called “salt-depositing phenomenon” is pointed out. Thatis, when a polyvinyl chloride-based resin is used as part of a binder,crystals of NaCl deposit on the surface of a magnetic recording layerwhich invites D.O. (dropout) to thus damage the quality of magnetictapes. Moreover, there are data showing that the above-mentionedmentioned phenomenon becomes a cause of “blocking” which is acharacteristic of video tapes (Blocking is a phenomenon that whenrunning of a video tape was stopped by power failure or the like withthe video tape being loaded on a video deck under high temperature andhigh humidity conditions, the magnetic recording layer is held to beattached to an upper cylinder of the video deck, in consequence, part ofthe magnetic recording layer peels off.). Thus, the reduction of theresidual Na⁺ contained in the magnetic powders has been long-waited.“Furthermore, Japanese Patent No. 2641662 discloses on page 2, column 3,line 38 to column 4, line 14; ” . . . fatty acids react with alkalimetals such as sodium and potassium which are impurities of carbonblackto thereby form alkali metal salts of the fatty acids. These alkalimetal salts of the fatty acids are insoluble in an organic solventpowder of these alkali metal salts of the fatty acids deposit on thesurface of the magnetic recording layer to thus become a cause ofdropout. “ . . . The decomposition amount of the organic solvent isproportional to the amount of the alkali metals such as Na and K . . .The decomposition products of the organic solvent lower adsorptivity ofa binder resin to the surface of an inorganic fine particle filter toresult in a decrease in mechanical strength of a coated film. Inaddition, storage stability as a coating composition deteriorates.”

It has been reported that an improvement in storage stability of amagnetic recording medium is tried by reducing a water-soluble sodiumsalt contained in a magnetic recording medium or in non-magnetic ofmagnetic particles added to the magnetic recording medium (JapanesePatent Non-examined Publication (Kokai) Nos. 62-209726, 62-209806,9-22524, 9-147350, 9-231546, 9-170003, 10-177714, 10-198943, JapanesePatent Examined Publication (Kakoku) No. 7-82638, Japanese Patent No.2641662, etc.).

As is discussed above, non-magnetic particles for a non-magneticundercoat layer have been strongly demanded which are capable ofproviding a thin magnetic recording layer having a smooth surface anduniform thickness when the magnetic recording layer is formed on thenon-magnetic undercoat layer obtained by dispersing the non-magneticparticles in a vehicle, and further, capable of providing a magneticrecording medium excellent in electromagnetic performance and storagestability, but such non-magnetic particles have not been hithertoobtained.

That is, in the above-mentioned Japanese Patent Non-examined Publication(Kokai) No. 9-147350, it is described that the amount of alkali metalscontained in non-magnetic particles of a non-magnetic layer is less than1500 ppm. However, as will be described later as comparative examples,when the non-magnetic particles contain approximately 1500 ppm of alkalimetals, dispersibility in a vehicle is poor because of high desorptionratio of resin, and hence, a magnetic recording medium obtained byemploying the non-magnetic particles as ones for a non-magneticundercoat layer of a magnetic recording medium is weak in strength of acoated film and storage stability is not said to be satisfactory.Moreover, as methods for production of non-magnetic particles havingalkali metals of less than 1500 ppm, a method for employing as an alkaliaqueous solution, for example, an ammonium aqueous solution notcontaining alkali metals, and a method for carrying out sufficientwashing after the completion of production or before the finalheat-treatment are described. However, according to these methods, thetotal amount of sodium can only be reduced to approximately 100 ppm asdescribed in the publication and can not be reduced to 50 ppm or less.Thus, when such non-magnetic particles are used as ones for anon-magnetic undercoat layer of a magnetic recording medium, it cannotbe said that the storage stability of the obtained magnetic recordingmedium is satisfactory.

In the above-mentioned Japanese Patent Non-examined Publication Nos.9-22524 and 9-170003, it is described that a soluble sodium contained innon-magnetic particles of a non-magnetic layer is 0-150 ppm or 300 ppmor less. However, as will be described later as comparative examples,according to these methods, a soluble sodium can be reduced toapproximately 45 ppm but a difficulty-soluble sodium is contained in anamount of approximately 300 ppm. Since the difficultly-soluble sodiumconverts into soluble sodium through moisture contained in air or acoated film and to come cut to deposit, when the non-magnetic particlesare used as ones for non-magnetic undercoat layer of a magneticrecording medium, the storage stability of the magnetic recording mediumcar not be said to be satisfactory.

SUMMARY OF THE INVENTION

An object of the present invention is to provide non-magnetic particlesfor a non-magnetic undercoat layer of a magnetic recording medium whichcan provide a thin magnetic recording layer having a smooth surface anda uniform thickness when a magnetic recording layer is formed on thenon-magnetic undercoat layer obtained by dispersing the non-magneticparticles in a vehicle, and which can provide a magnetic recordingmedium excellent in electromagnetic performance and storage stability.

Another object of the present invention is to provide a method forproducing the non-magnetic particles for a non-magnetic undercoat layerof a magnetic recording medium.

Still another object of the present invention is to provide a magneticrecording medium which is excellent in electromagnetic performance andstorage stability.

Further objects and advantages of the present invention will be apparentfrom the detailed description below.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is, in a first aspect, to provide particles for anon-magnetic undercoat layer of a magnetic recording medium, whichcomprises acicular hematite particles having an average major axialdiameter of not more than 0.3 μm, a geometrical standard deviation ofthe major axial diameter of not more than 1.50 and a BET specificsurface area of no: less than 40 m²/g, and containing a total amount ofsodium of not more than 50 ppm calculated as Na.

As a preferable embodiment, the acicular hematite particles are coatedwith at least one selected from the group consisting of an aluminiumhydroxide, an aluminium oxide, a silicon hydroxide and a silicon oxide.

As a further preferable embodiment, an S_(BET)/S_(TEM) defined by aratio of a specific surface area (S_(BET)) measured by a BET method anda surface area (S_(TEM)) calculated from the major axial diameter andthe minor axial diameter, measured from the particles in an electronmicroscopic photograph is 0.5 to 2.5.

The present invention is, in a second aspect, to provide a method forproducing particles for a non-magnetic undercoat layer of a magneticrecording medium, which comprises the steps of:

dehydrating acicular goethite particles with the surfaces coated with asintering-preventing agent to form acicular hematite particles,

reducing the acicular hematite particles at a temperature of 250 to 600°C. under a reducing atmosphere to form acicular magnetite particles,

washing with pure water and drying the acicular magnetite particles,

oxidizing the acicular magnetite particles at a temperature of 650 to850° C. under an oxidizing atmosphere and washing with pure water anddrying the resulting high-density acicular hematite particles.

The present invention is, in a third aspect, to provide a method forproducing particles for a non-magnetic undercoat layer of a magneticrecording medium, which comprises the steps of:

dehydrating acicular goethite particles to form acicular hematiteparticles,

coating the surfaces of the acicular hematite particles with asintering-preventing agent,

reducing the acicular hematite particles at a temperature of 250 to 600°C. under a reducing atmosphere to form acicular magnetite particles,

washing with pure water and drying the acicular magnetite particles,

oxidizing the acicular magnetite particles at a temperature of 650 to850° C. under an oxidizing atmosphere, and

washing with pure water and drying the resulting high-density acicularhematite particles.

As a preferable embodiment in the above-mentioned two methods, theacicular magnetite particles and high-density acicular hematiteparticles are wet-pulverized prior to washing with pure water.

As a further preferable embodiment, the high-density acicular hematiteparticles are coated with at least one selected from the groupconsisting at an aluminium hydroxide, an aluminium oxide, a siliconhydroxide and a silicon oxide by treating the particles with an aqueoussolution containing an aluminium compound, a silicone compound or theboth compounds.

The present invention is, in a fourth aspect, to provide a magneticrecording medium comprising a non-magnetic base film, a non-magneticundercoat layer formed or: the non-magnetic base film, comprisingnon-magnetic particles and a binder resin, and a magnetic recordinglayer formed on the non-magnetic undercoat layer, comprising magneticparticles and a binder resin, the improvement wherein the non-magneticparticles comprises particles for a non-magnetic undercoat layer asmentioned above.

Hereinafter, the present invention will be explained in detail.

First, particles for a non-magnetic undercoat layer according to thepresent invention will be explained.

The particles for a nor-magnetic undercoat layer according to thepresent invention comprises acicular hematite particles having anaverage major axial diameter of not more than 0.3 μm, a geometricalstandard deviation of the major axial diameter of not more than 1.50 anda BET specific surface area of not less than 40 m²/g, and containing thetotal amount of sodium of not more than 50 ppm calculated as Na.

The word “acicular” of the acicular hematite particles herein includesnot only an acicular shape, literally, but, for example, a spindle shareand a rice shape.

The average major axial diameter of the acicular hematite particles fora non-magnetic undercoat layer is not more than 0.3 μm, preferably 0.005to 0.3 μm. If it is less than 0.005 μm, the dispersion in a vehicletends to be difficult upon the production of a non-magnetic coatingcomposition because of increased intermolecular force caused by fineparticles. If it is more than 0.3 μm, the surface smoothness of a coatedfilm tends to be deteriorated because of the increased particle size.The average major axial diameter is more preferably 0.02 to 0.2 μm whenconsidering the dispersibility in the vehicle and the surface smoothnessof a coated film.

The geometrical standard deviation of the major axial diameter of theacicular hematite particles is not more than 1.50. If it is more than1.50, the coarse particles give an adverse effect to the surfacesmoothness. It is more preferably not more than 1.40, still morepreferably not more than 1.35, when considering the surface smoothnessof a coated film. Further, if the industrial productivity is taken intoconsideration, the lower limit thereof is approximately 1.01.

The BET specific surface area of the acicular hematite particles is notless than 40 m²/g, preferably to 150 m²/g. More preferably, it is 45 to100 m²/g, still more preferably 50 to 80 m²/g for the same reasons as inthe upper and lower limits of the above-mentioned average major axialdiameter.

The total amount of sodium as calculated as Na contained in the acicularhematite particles is not more than 50 ppm. If it is more than 50 ppm,difficulty-soluble sodium salts contained in the particles are convertedinto soluble sodium salts through moisture contained in air and a coatedfilm to thus deposit on the surfaces of the particles, then the solublesodium salts react with fatty acids added to the coated film to thusgenerate metal salts of the fatty acids, which gradually lowerselectromagnetic performance of the obtained magnetic recording mediumand as a result, the storage stability becomes worse. In some cases, thedispersibility of the acicular hematite particles in a vehicle is liableto be deteriorated and under high humidity surroundings, in particular,an efflorescence phenomenon sometimes occurs on the surface of themagnetic recording medium. If the storage stability of the magneticrecording medium is taken into consideration, the total amount of sodiumas calculated as Na contained in the acicular hematite particles ispreferably not more than 45 ppm, more preferably not more than 40 ppm,still more preferably not more than 35 ppm. The lower limit isapproximately 0.01 ppm, if the industrial productivity is taken intoconsideration.

The storage stability of the acicular hematite particles (the content ofthe soluble sodium salts calculated as Na contained in the acicularhematite particles after left to stand for 14 days under surroundings ata temperature of 60° C. and a relative humidity of 90%) is preferablynot more than 40 ppm. If it exceeds 40 ppm, the fatty acids added to acoated film and the soluble sodium salts react to generate metal saltsof the fatty acids and they gradually lower the electromagneticperformance of a magnetic recording medium so that the storage stabilityis deteriorated. Under high humidity surroundings, in particular, anefflorescence phenomenon sometimes occurs on the surface of the magneticrecording medium. If the storage stability of the magnetic recordingmedium obtained is taken into consideration, it is preferably not morethan 35 ppm, more preferably not more than 30 ppm, still more preferablynot more than 25 ppm. The lower limit is, if the industrial productivityis taken into consideration, approximately 0.01 ppm.

The average minor axial diameter of the acicular hematite particles ispreferably 0.0025 to 0.15 μm and the aspect ratio (ratio of averagemajor axial diameter/average minor axial diameter) is preferably notless than 2.

The average minor axial diameter is more preferably 0.01 to 0.10 μm forthe same reasons as in the lower and upper limits of the above-mentionedaverage major axial diameter.

The upper limit of the aspect ratio is preferably 20. If it is more than20, the particles twine in a vehicle upon the preparation of anon-magnetic coating composition, which often lowers dispersibility orincreases viscosity. If it is less than 2. the stiffness of a coatedfilm obtained becomes insufficient. If the dispersibility in the vehicleand the stiffness of the coated film are taken into consideration, theaspect ratio is more preferably 3 to 10.

The acicular hematite particles have preferably a higher densification.The degree of densification is defined by the ratio of S_(BET)/S_(TEM)in which the specific surface area S_(BET) is measured by a BET methodand the surface area S_(TEM) is calculated from the major axial diameterand the minor axial diameter, which were measured from the particles inan electron microscopic photograph, and have preferably a value of 0.5to 2.5.

If the S_(BET) /S_(TEM) value is less than 0.5, the acicular hematiteparticles are densified, but the particle size increases because ofsintering of the particles and thus a coated film excellent in thesurface smoothness can not be obtained. If it is more than 2.5, thedensification is rot sufficient to allow many pores to exist in theinteriors or on the surfaces of the particles so that the dispersibilityin a vehicle may be insufficient. If the dispersibility in the vehicleand the surface smoothness of the coated film are taken intoconsideration, its value is preferably 0.7 to 2.0, more preferably 0.8to 1.6.

The hematite particles are low in resin desorption ratio and if it isrepresented by a value obtained by the method as will be describedlater, its value is preferably not more than 30%. If it is more than30%, the dispersibility in a vehicle and the dispersion stability notonly lower, but the stiffness of the coated film lowers. If thedispersibility, dispersion stability in the vehicle and the stiffness ofthe coated film are taken into consideration it is more preferably notmore than 25%, still more preferably not more than 20%.

The surfaces of the acicular hematite particles may, if necessary, becoated with at least one selected from the group consisting of analuminium hydroxide, an aluminium oxide, a silicon hydroxide and asilicon oxide (hereinafter referred to coating substance). The acicularhematite particles coated are improved in the dispersibility in thevehicle as compared with non-coated ones.

The amount of the coating substance is preferably 0.01 to 50% by weightcalculated as Al and/or SiO₂ on the basis of the acicular hematiteparticles. If it is less than 0.01 by weight, thedispersibility-improving effect resulting from the coating is difficultto be obtained, and if it is more than 50% by weight, the coating effectarrives at the saturated level and thus the coating more than necessaryis meaningless. If the dispersibility in the vehicle and the industrialproductivity are taker into consideration, it is more preferable 0.05 to20% by weight.

The acicular hematite particles coated with the coating substance havealmost the same particle size, aspect ratio, BET specific surface area,geometrical standard deviation, S_(BET)/S_(TEM) and the total amount ofsodium as the non-coated acicular hematite particles.

Next, a magnetic recording medium according to the present inventionwill be explained.

The magnetic recording medium according to the present inventioncomprises a non-magnetic base film, a non-magnetic undercoat layerformed on the ran-magnetic base film and a magnetic recording layerformed on the non-magnetic undercoat layer.

As the non-magnetic base film, synthetic resins such as polyethyleneterephthalate, polyethylene, polypropylene, polycarbonate, polyethylenenaphthalate, polyamide, polyamideimide and polyimide, foil and plate ofa metal such as aluminium and stainless steel, and various kinds ofpaper, which are widely used for the production of a magnetic recordingmedium can be used. The thickness of the non-magnetic base film ispreferably 1.0 to 300 μm, more preferably 2.0 to 200 μm though variabledepending on the material employed. In the case of a magnetic disc,polyethylene terephthalate is ordinarily used as the non-magnetic basefilm, and its thickness is ordinarily 50 to 300 μm, preferably 60 to 200μm. In the case of a magnetic tape, when polyethylene terephthalate isused, its thickness is ordinarily 3 to 100 μm, preferably 4 to 20 μm,and when polyethylene naphthalate is used, its thickness is ordinarily 3to 50 μm, preferably 4 to 200 μm, and when the polyamide is used, itsthickness is ordinarily 1 to 100 μm preferably 3 to 7 μm.

The non-magnetic undercoat layer of a magnetic recording medium in thepresent invention comprises acicular hematite particles which areparticles for a non-magnetic undercoat layer and a binder resin.

As the binder resin, various hinder resins which are widely used for theproduction of a magnetic recording medium can be used.

Examples of the binder resin are vinyl chloride-vinyl acetate copolymer,urethane resin, vinyl chloride-vinyl acetate-maleic acid terpolymer,urethane elastomer, butadiene-acrylonitrile copolymer, polyvinylbutyral, cellurose derivatives such as nitrocellulose, polyester resin,synthetic rubber resins such as polybutadiene, epoxy resin, polyamideresin, polyisocyanates, electron beam-curing acryl urethane resin andmixtures thereof. Each of these binder resins may contain a functionalgroup such as —OH, —COOH, —SO₃M, —OPO₂M₂ and —NH₂, wherein M representsH, Na or K. If the dispersibility of the acicular hematite particles istaken into consideration, the binder resin containing —COOH or —SO₃M asthe functional group is preferable.

The blending ratio of the acicular hematite particles and the binderresin is 5 to 2000 parts by weight, preferably 100 to 1000 parts byweight based on 100 parts by weight of the binder resin. If the acicularhematite particles are less than 5 parts by weight, the acicularhematite particles contained in a non-magnetic coating composition istoo short to obtain a continuously dispersed layer of the acicularhematite particles, and thus the surface smoothness and the stiffness ofthe non-magnetic undercoat layer can not be said to be sufficient. If itis more than 2000 parts by weight, the acicular hematite particles aretoo plentiful with respect to the binder resin to sufficiently dispersein a non-magnetic coating composition and as a result, the coated filmwith the sufficient surface smoothness is difficult to be obtained.Moreover since the acicular hematite particles are not bound adequatelyby the binder resin, the obtained coated film is apt to be brittle.

The coated thickness of the non-magnetic undercoat layer is preferably0.2 to 10.0 μm. If it is less than 0.2 μm the surface roughness of thenon-magnetic base film is not improved sufficiently and the stiffness isalso apt to be insufficient. If the thinning of a magnetic recordingmedium and the stiffness of the coated film are taken intoconsideration, the coated thickness is more preferably 0.5 to 5.0 μm.

Meanwhile, it is possible to add to the non-magnetic undercoat layer,additives such as a lubricant, a polishing agent and an antistatic agentwhich are ordinarily used for the production of a magnetic recordingmedium.

The gloss of the non-magnetic undercoat layer using acicular hematiteparticles, the surfaces of which are not coated with the above-mentionedcoating substance such as aluminium hydroxide is 190 to 300%, preferably193 to 300%, more preferably 195 to 300%, the surface roughness Rathereof is 0.5 to 8.0 nm, preferably 0.5 to 7.5 nm, more preferably 0.5to 7.0 nm, and the stiffness (Young's modulus; relative value) thereofis 125 to 160 preferably 128 to 160.

The gloss of the non-magnetic undercoat layer using the acicularhematite particles, which are coated with the above-mentioned coatingsubstance such as aluminium hydroxide is 192 to 300%, preferably 195 to300%, more preferably 200 to 300%, the surface roughness Ra thereof is0.5 to 7.8 nm, preferably 0.5 to 7.0 nm, more preferably 0.5 to 6.8 nm,and the stiffness of the coated film (Young's modulus; relative value)is 128 to 160, preferably 130 to 160.

The magnetic recording layer in the present invention comprises magneticparticles and a binder resin.

As the magnetic particles, Co-coated magnetic iron oxide particlesobtained by coating magnetic iron oxide particles such as maghemiteparticles (γ−Fe₂O₃) and magnetite particles (FeOx·Fe₂O₃, O<x≦1) with Coor Co and Fe, Co-coated magnetic iron oxide particles obtained by addingthe different kinds of elements other than iron, such as Co, Al, Ni, P,Zn, Si, B and a rare earth element to the above-mentioned Co-coatedmagnetic iron oxide particles, acicular metal magnetic particles mainlycontaining iron, acicular iron-based alloy magnetic particles containingelements other than iron, such as Co, Al, Ni, P, Zn, Si and B,plate-like magnetoplumbite ferrite particles containing Ba, Sr, Ba—Sr,and plate-like magnetoplumbite ferrite particles containing one or moreof a coercive force-reducing agent selected from divalent andtetravalent metals such as Co, Ni, Zn, Mn, Mg, Ti, Sn and Zr areexemplified, and these may be used singly or in combination of two ormore.

Meanwhile, under the consideration of high-density recording of recentmagnetic recording medium, among the above magnetic particles, theacicular metal magnetic particles mainly containing iron and theacicular iron-based alloy magnetic particles containing elements otherthan iron, such as Co, Al, Ni, P, Zn, Si, B and a rare earth metalelement are preferable.

The average major axial diameter of the magnetic particles (averageparticle size in the case of a plate-dike particle) is 0.01 to 0.50 μm,preferably 0.03 to 0.30 μm, The shape of the magnetic particles isacicular or plate-like. The word “acicular” herein means not only anacicular shape literally, but a spindle shape and a rice shape.

When the shape of the magnetic particles is acicular, the aspect ratiois not less than 3, preferably not less than 5. If the dispersibility ina vehicle upon the preparation of a magnetic coating composition istaken into consideration, the upper limit is approximately 15,preferably approximately 10.

When the shape of the magnetic particles is plate-like, the plate ratio(ratio of an average particle size/an average thickness) is not lessthan 2, preferably not less than 3. If the dispersibility in a vehicleupon the preparation of a magnetic coating composition, the upper limitis approximately 20, preferably approximately 15.

As the magnetic properties, the coercive force is 500 to 3200 Oe,preferably 550 to 3200 Oe, the saturation magnetization is 50 to 170emu/g, preferably 60 to 170 emu/g. If the properties such ashigh-density recording of the magnetic recording medium are taken intoconsideration, the coercive force is more preferably 900 to 3200 Oe andthe saturation magnetization is more preferably 70 to 170 emu/g.

As the binder resin, the binder resin used for forming theabove-mentioned non-magnetic undercoat layer can be used.

The blending ratio of the magnetic particles and the binder resin is 200to 2000 parts by weight, preferably 300 to 1500 parts by weight of themagnetic particles based on 100 parts by weight of the binder resin.

The coated thickness of the magnetic recording layer formed on thenon-magnetic undercoat layer is 0.01 to 5.0 μm. If it is less than 0.01μm, the uniform coating is difficult and the uneven coating tends tooccur. If it is more than 5.0 μm, an influence of theself-demagnetization becomes large and the desired electromagneticperformance is difficult to be obtained. The coated thickness ispreferably 0.05 to 1.0 μm.

It is possible to add to the magnetic recording layer, additives such asa lubricant, a polishing agent and an antistatic agent which arenormally used.

The magnetic recording medium according to the present invention, whenthe Co-coated magnetic iron oxide particles are used as the magneticparticles, has a coercive force of 300 to 1500 Oe preferably 550 to 1330Oe, a squareness (residual flux density Br/saturation flux density Bm)of 0.85 to 0.95, preferably 0.86 to 0.95, a gloss of a coated film of130 to 200%, preferably 140 to 200%, a surface roughness Ra of a coatedfilm of not more than 12.0 nm, preferably 2.0 to 11.0 nm, morepreferably 2.0 to 10.0 nm, a Young's modulus (relative value to acommercially available video tape: AV T-120 produced by Victor Companyof Japan, Limited) of 125 to 160, preferably 130 to 160. As theelectromagnetic performance, the output at 4 MHz is not less than +0.5dB as compared with that of the standard tape obtained by using as thenon-magnetic particles for a non-magnetic undercoat layer, non-magneticparticles other than those of the present invention and using the samemagnetic particles for a magnetic recording layer as in the presentinvention, the output drop at 4 MHz after stored at a temperature of 60°C. and a relative humidity of 90% for 14 days is not more than 1.0 dB,and the stain on a head after 30-minute running of a magnetic tape is 2,preferably 1, according to the evaluation method as will be describedlater.

The magnetic recording medium according to the present invention, whenthe acicular metal magnetic particles mainly containing iron or aciculariron-based alloy magnetic particles are used as the magnetic particles,has a coercive force of 800 to 3200 Oe, preferably 900 to 3200 Oe, asquareness (residual flux density Br/saturation flux density Bm) of 0.85to 0.95, preferably 0.86 to 0. 95, a gloss of a coated film of 180 to300%, preferably 190 to 300%, a surface roughness Ra of a coated film ofnot more than 12.0 nm, preferably 2.0 to 11.0 nm, more preferably 2.0 to10.0 nm, a Young's modulus (relative value to a commercially availablevideo tape: AV T-120 produced by Victor Company of Japan, Limited) of124 to 160, preferably 128 to 160. As the electromagnetic performance,the output at 7 MHz is not less than +0.5 dB as compared with that ofthe standard tape obtained by using as the non-magnetic particles for anon-magnetic undercoat layer, the non-magnetic particles other thanthose of the present invention and using the same magnetic particles fora magnetic recording layer as in the present invention, the output dropat 7 MHz after stored at a temperature of 60° C. and a relative humidityof 90% for 14 days is not more than 1.0 dB, and the stain on a headafter 30-minute running of a magnetic tape is 2, preferably 1, accordingto the evaluation method as will be described later.

The magnetic recording medium according to the present invention, whenthe plate-like magnetoplumbite ferrite particles are used as themagnetic particles, has a coercive force of 800 to 3200 Oe, preferably900 to 3200 Oe, a squareness (residual flux density Br/saturation fluxdensity Bm) of 0.85 to 0.95, preferably 0.86 to 0. 95, a gloss of thecoated film of 160 to 300%, preferably 170 to 300%, a surface roughnessRa of a coated film of not more than 12.0 nm, preferably 2.0 to 11.0 nm,more preferably 2.0 to 10.0 nm, a Young's modulus (relative value to acommercially available video tape: AV T-120 produced by Victor Companyof Japan, Limited) of 124 to 160, preferably 128 to 160. As theelectromagnetic performance, the output at 7 MHz is not less than +0.5dB as compared with that of the standard tape obtained by using as thenon-magnetic particles for a non-magnetic undercoat layer, non-magneticparticles other than those of the present invention and using the samemagnetic particles for a magnetic recording layer as in the presentinvention, the output drop at 7 MHz after stored at a temperature of 60°C. and a relative humidity of 90% for 14 days is not more than 1.0 dB,and the stain on a head after 30-minute running of a magnetic tape is 2,preferably 1, according to the evaluation method as will be describedlater.

Next, the method for producing the acicular hematite particles for anon-magnetic undercoat layer according to the present invention will beexplained.

The acicular hematite particles are obtained by dehydrating by heatingacicular goethite particles obtained by an ordinary method to thusobtain acicular hematite particles, reducing the obtained acicularhematite particles to thus obtain acicular magnetite particles, removingby washing sodium deposited on the surfaces, then oxidizing.

The goethite particles as the starting material in the present inventioncan be obtained by an ordinary method, i.e., conducting an oxidationreaction by passing an oxygen-containing gas such as air into asuspension containing iron-containing presipitates such as ironhydroxide and iron carbonate obtained by reacting an aqueous ferroussalt solution with an aqueous alkali hydroxide solution, an aqueousalkali carbonate solution or an aqueous alkali hydroxide and an alkalicarbonate solution.

Meanwhile, it is possible to add, during the course of the syntheticreaction of the acicular goethite particles, the different elements suchas Ni, Zn, P, Si and Al which are added to enhance the characteristicssuch as the major axial diameter, minor axial diameter and aspect ratio.

Prior to the reduction by heating, it is necessary to conduct thecoating treatment with a sintering-preventing agent. The coatingtreatment is performed by adding the sintering-preventing agent into anaqueous suspension containing acicular goethite particles or low-densityhematite particles obtained by dehydrating by heating the aciculargoethite particles at a temperature of 250 to 500° C., mixing bystirring, followed by filtration, washing and drying.

As the sintering-preventing agent, it is possible to usesintering-preventing agents which are ordinarily used. Examples arephosphorus compounds such as sodium hexamethaphosphate, polyphosphoricacid and orthophosphoric acid, silicon compounds such as water glass #3,sodium orthosilicate, sodium metasilicate and colloidal silica, boroncompounds such as boric acid, aluminium compounds such as aluminiumacetate, aluminium sulfate, aluminium chloride, aluminium nitrate andsodium aluminate, and titanium compounds such as titanyl sulfate. Amongthese compounds, orthophosphoric acid, colloidal silica, boric acid andaluminium acetate are preferable. These are used singly or incombination of

The coated amount of the sintering-preventing agent is preferably 0.05to 10% by weight, more preferably 0.1 to 5% by weight in an amountcalculated as P, SiO₂, B, Al or Ti based on the total weight of theparticles.

The total sodium content calculated as Na of the acicular goethiteparticles obtained is 600 to 3000 ppm, the content of soluble sodiumsalts calculated as Na is 300 to 1500 ppm, the average major axialdiameter is 0.01 to 0.3 μm, the average minor axial diameter is 0.001 to0.15 μm, the aspect ratio is 3 to 25, the geometrical standard deviationof the major axial diameter is not more than 1.5 and the BET specificsurface area is 50 to 250 m²/g.

The acicular goethite particles coated with the sintering-preventingagent is dehydrated by heating at a temperature of 250 to 500° C. toobtain the low-density acicular hematite particles.

The low-density acicular hematite particles are 600 to 3000 ppm in totalcontent of sodium salts calculated as Na, 500 to 2000 ppm in content ofsoluble sodium salts calculated as Na, 0.005 to 0.3 μm in average majoraxial diameter, 0.0025 to 0.15 μm in average minor axial diameter, 3 to20 in aspect ratio, not more than 1.5 in geometrical standard deviationof the major axial diameter, 70 to 350 m²/g in BET specific surfacearea, and 2.5 to 6. 0 in degree of densification S_(BET)/S_(TEM).

If the heating temperature is lower than 250° C., the dehydrationreaction takes a long time and thus it is not preferable. If it ishigher than 500° C., the dehydration reaction takes place rapidly tothus result in deformation in particle shape and sintering among theparticles, and thus it is not preferable.

The acicular hematite particles obtained by the heat-treatment arelow-density particles having a lot of dehydrated pores, which aredehydrated from the acicular goethite particles, and the BET specificsurface area is approximately 1.2 to 2 times that of the aciculargoethite particles as the starting particles.

The low-density hematite particles obtained are then subjected to thereduction treatment at a temperature of 250 to 600° C. under a reducingatmosphere to thus form low-density acicular magnetite particles so thatsodium compounds contained in the interiors of particles may bedeposited on the surfaces of the particles.

If the temperature is lower than 250° C., the reduction reaction takes alonger time and thus it is not preferable. If it is higher than 600° C.,the reduction reaction takes place rapidly to thus invite thedeformation in particle shape and sintering among the particles and thusit is not preferable.

The low-density magnetite particles obtained are 600 to 3000 ppm intotal amount of sodium calculated as Na, 600 to 3000 ppm in solublesodium salts calculated as Na, 0.01 to 0.3 μm in average major axialdiameter, 0.005 to 0.15 μm in average minor axial diametel, 2 to 20 inaspect ratio, not more than 1.5 in geometrical standard deviation of themajor axial diameter, 40 to 250 m²/g in BET specific surface area, and2.5 to 5.0 in degree of densification S_(BET)/S_(TEM).

The low-density acicular magnetite particles obtained are roughlypulverized into coarse particles by a dry method and formed into aslurry. The slurry is then pulverized by a wet method to thus remove thecoarse particles. The wet method pulverization is conducted by the useof ball mill, a sand grinder, a Daino mill, a colloid mill or the liketo such an extent that coarse particles of not less than 44 μm are notmore than 10%, preferably not more than 5%, more preferably 0%. If thecoarse particles of not less than 44 μm remain in an amount of more than10%, the sufficient removal effect of the deposited sodium component atthe washing step is not obtained.

The low-density acicular magnetite particles obtained by pulverizing thecoarse particles by the wet pulverization method are filtered and washedwith pure water by an ordinary method and the sodium components areremoved by washing, then dried.

As the method for washing with water, any known methods which areindustrially used, such as a decantation method, a dilution method usinga filter thickner and a method of passing water into a filter press areemployed.

The low-density acicular magnetite particles after washing with purewater are 50 to 1500 ppm in total sodium content calculated as Na, 30 to300 ppm in soluble sodium salts calculated as Na, 0.01 to 0.3 μm inaverage major axial diameter, 0.005 to 0.15 μm in average minor axialdiameter, 2 to 20 in aspect ratio, not more than 1.5 in geometricalstandard deviation of the major axial diameter, 40 to 250 m²/g in BETspecific surface area, and 2.5 to 5. 0 in degree of densificationS_(SET)/S_(TEM).

Then, the low-density acicular magnetite particles are subjected to theoxidation reaction at a temperature of 650 to 850° C. under an oxidizingatmosphere to thus obtain high-density acicular hematite particles byway of acicular maghemite particles.

If the temperature is lower than 650° C., since the acicular maghemiteparticles are mixed in the acicular hematite particles, the obtainedacicular particles have magnetism. Moreover, the acicular hematiteparticles have a large number of dehydrated pores inside the particlesor on the surfaces of the particles due to the insufficientdensification. As a result, the dispersibility in a vehicle isinsufficient and a coated film with the surface smoothness is difficultto be obtained. If the temperature is higher than 850° C., though theacicular hematite particles are sufficiently densified, since sinteringamong the particles takes place, the particle size increases and thus acoated film with the surface smoothness is difficult to be obtained.

The high-density acicular hematite particles are 50 to 1500 ppm in totalsodium content calculated as Na, 50 to 1500 ppm in soluble sodium saltscalculated as Na, 0.005 to 0.3 μm in average major axial diameter,0.0025 to 0.15 μm in average minor axial diameter, 2 to 20 in aspectratio, not more than 1.5 in geometrical standard deviation of the majoraxial diameter, 40 to 250 m²/g in BET specific surface area, and 2.5 to5.0 in degree of densification S_(SET)/S_(TEM).

The obtained high-density acicular hematite particles are, in the samemanner as in the washing step of the low-density acicular magnetiteparticles, pulverized by a dry method and formed into a slurry,thereafter pulverized by a wet method, filtered and washed with purewater by an ordinary method, whereby the sodium component deposited onthe surfaces of the particles are removed by washing and dried.

In order to improve the affinity with the binder resin to enhance thedispersibility, the so obtained acicular hematite particles may, ifnecessary, be coated with at least one selected from an aluminiumhydroxide, an aluminium oxide, a silicon hydroxide and a silicon oxide.

The coating treatment is conducted by adding the aluminium compound orthe silicon compound or the both compounds to a suspension containingthe acicular hematite particles obtained by washing with water after theoxidation reaetion, followed by mixing and stirring, and furtheradjusting a pH value, if required, followed by filtration, washing withwater, drying and pulverization. Deaeration and a compression treatmentor the like may be further conducted, if necessary.

As the aluminium compound in the present invention, aluminium salts suchas aluminium acetate, aluminium sulfate, aluminium chloride andaluminium nitrate, aluminium compounds such as aluminium hydroxide,aluminium oxide and alumina sol are usable.

The amount of the aluminium compound calculated as Al is 0.01 to 50% byweight, preferably 0.05 to 20% by weight based on the weight of theacicular hematite particles. If it is less than 0.01% by weight, theimproving effect of dispersing in a vehicle is not obtained. If it ismore than 50% by weight, the dispersibility-improving effect arrives atthe saturation level and thus addition more than necessary ismeaningless.

As the silicon compound in the present invention, silicates such aspotassium silicate, silicon compounds such as silicon hydroxide andsilicon oxide and collidal silica are usable.

The amount of the silicon compound added calculated as SiO₂ is 0.01 to50% by weight, preferably 0.05 to 20% by weight based on the weight ofthe acicular hematite particles. If it is less than 0.01% by weight, theimproving effect of dispersing in a vehicle is not obtained. If it ismore than 50% by weight, the dispersibility-improving effect arrives atthe saturation level and thus addition more than necessary ismeaningless.

When the aluminium compound and the silicon compound are mixed, thetotal amount calculated as Al and SiO₂ is 0.01 to 50% by weight, morepreferably 0.05 to 20% by weight.

Next, the method for producing a non-magnetic substrate for a magneticrecording medium having a non-magnetic undercoat layer in the presentinvention will be explained.

The non-magnetic substrate for a magnetic recording medium in thepresent invention is obtained by coating a non-magnetic coatingcomposition containing acicular hematite particles, a binder resin and asolvent, on a non-magnetic base film to thus form a non-magneticundercoat lyer thereon, and drying.

As the solvent, solvents which are ordinarily used for the production ofa magnetic recording medium such as methyl ethyl ketone, toluene,cyclohexanone, methylisobutyl, ketone and tetrahydrofuran areexemplified and these are used singly or in combination of two or more.

The amount of the solvent in the non-magnetic coating composition ispreferably 50 to 95 parts by weight based on 100 parts by weight of thenon-magnetic coating composition. If it is less then 50 parts by weight,the viscosity of the non-magnetic coating composition obtained becomestoo high to make the coating difficult. If it is more than 95 parts byweight, the volatile amount of the solvent becomes too large and thus itis disadvantageous industrially.

The non-magnetic coating composition in the present invention isexcellent in dispersion stability and the change ratio in gloss showingthe dispersion stability of the non-magnetic coating composition, whichis obtained by a measurement method as will be mentioned later is notmore than 5%.

Next, the method for producing a magnetic recording medium according tothe present invention will be explained.

The magnetic recording medium of the present invention is obtained bycoating a magnetic coating composition containing magnetic particles, abinder resin and a solvent on the non-magnetic substrate having thenon-magnetic undercoat layer to thus form a magnetic recording layerthereon, and drying.

As the solvent, the above-mentioned solvents as used in the non-magneticpaint are usable.

The amount of the solvent is preferably 50 to 95 parts by weight basedon 100 parts by weight of the magnetic coating composition. If it isless than 50 parts by weight, the viscosity of the obtained magneticcoating composition becomes too high to make the coating difficult. Ifit is more than 95 parts by weight, the volatile amount of the solventbecomes too large and thus it is disadvantageous industrially.

The most important point is that when the high-density hematiteparticles having an excellent dispersibility in the vehicle andcontaining not more than 50 ppm of the total content of sodium are usedas the non-magnetic particles for a non-magnetic undercoat layer, it ispossible to enhance the surface smoothness and the strength of thenon-magnetic undercoat layer due to the excellent dispersibility in thebinder resin, and that when a magnetic recording layer is formed on thenon-magnetic undercoat layer, it is possible not only to form a thinlayer having the smooth surface and the uniform thickness, but to obtaina magnetic recording medium having an excellent electromagneticperformance and storage stability.

The reason why the total content of sodium contained in the acicularhematite particles can be reduced to not more than 50 ppm is presumablyconsidered that since difficulty-soluble sodium salts fixed to crystalsinside the non-magnetic particles which can not be removed by anordinary washing deposit on the surfaces of the particles with themodification of the crystal form and convert into soluble sodium salts,it became possible to remove the sodium by an ordinary washing withwater.

The reason why the surface smoothness and the strength of thenon-magnetic undercoat layer are enhanced is presumably considered thatsince it is possible to sufficiently remove by washing with water thesodium salts which cause the high-density hematite particles toaggregate by firmly crosslinking, the aggregates are separated intosubstantially discrete particles, and since in the vehicle, theadsorption of the binder resin onto the surfaces of the acicularhematite particles is made directly through elements other than thesodium element, the desorption of the binder resin from the surfaces ofthe acicular hematite particles decreases, so that the dispersibility ofthe acicular hematite particles is improved.

The reason why the storage stability as well as the electromagneticperformance of the magnetic recording medium is enhanced is presumablyconsidered that since it is possible to reduce the total content ofsodium in the acicular hematite particles contained in the non-magneticundercoat layer to not more than 50 ppm, as a result, to reduce not onlythe suluble sodium salts on the surfaces of the particles, but thedifficulty-soluble sodium salts contained in the interiors of theparticles which deposit on the surfaces of the particles as converted tothe soluble sodium salt for some causes such as moisture in air, themetal salts of fatty acids synthesized by the reaction with fatty acidsadded to a coated film can be reduced.

Next, the typical embodiment of the present invention will be described.

The remaining amount of coarse particles on a sieve was measured bypassing through a 325 mesh-sieve (sieve opening: 44 μm) a slurrycontaining 100 g of the particles, the concentration of which wasprelimirarily measured after the wet-pulverization, and weighing theparticles which do not pass through the sieve.

The average major axial diameter and the average minor axial diameter ofthe particles are represented by an average values of 350 particles inan electron microscopic photograph (×30,000) enlarged 4-fold in thelongitudinal and transverse directions.

The aspect ratio of the particles was calculated from a ratio of theaverage major axial diameter to the average minor axial diameter.

The particle size distribution of the major axial diameter of theparticles is represented by a geometrical standard deviation obtained bythe following method. That is, the major axial diameters of 350particles in the enlarged electron microscopic photograph were measured.The actual major axial diameters of the particles and the accumulativenumber of particles were obtained from the calculation on the basis ofthe measured values. In a logarithmic-normal probability paper, themajor axial diameters were plotted at the same intervals on the abscissaand the accumulative number of particles passed through the sievebelonging to each interval of the major axial diameters was plotted bypercentage on the ordinate by a statistical technique. The major axialdiameters corresponding to the number of particles of 50% and 84.13%,respectively, were read from the graph, and the geometrical standarddeviation was determined from the following equation:

Geometrical standard deviation=(major axial diameter corresponding tothe accumulative number of particles of 84.13%)/(major axial diametercorresponding to the accumulative number of particles of 50%)(geometrical average diameters)

As the geometrical standard deviation becomes close to 1, the particlesize distribution of the average axial diameter becomes excellent.

The specific surface area was represented by a value measured by a BETmethod (nitrogen adsorption method).

The total content of sodium contained in the particles was measured bythe following method. That is, 1.000 g of the particles was charged intoa 200 ml beaker. Then, 25 ml of a 12 mol/l hydrochloric acid were addedand the particles were dissolved by heating with a lid of a watch glassput on a beaker. After cooling, the contents were transferred to a 500ml measuring flask and pure water was added accurately to a 500 mlsolution, and the total content of sodium in the solution was measuredby the use of Industively Coupled Plasma Atomic EmissionSpectrophotometer manufactured by Seiko Instruments Inc.

The content of the soluble salts was measured by the following method.That is, 5 g of the particles were charged into a 300 ml conical flask.100 ml of pure water were added, heated and boiled for 5 minutes, afterstoppering, cooled to room temperature. Then, pure water was added in anamount equivalent to the pure water lost by boiling, and afterstoppering, the contents in the conical flask were shaked for 1 minuteand left to stand for 5 minutes. The supernatant liquid was filtered bythe use of a No. 5 C filter paper and the content of Na⁺ in the filtratewas measured by the use of Inductively Coupled Plasma Atomic EmissionSpectrophotometer manufactured by Seiko Instruments Inc.

The storage stability of the acicular hematite particles was representedby a content of the soluble salts (calculated as Na) measured by thesame method as mentioned above, after left to stand at a temperature of60° C. and a relative humidity of 90% for 14 days.

The degree of densification of the particles is represented by a valueof S_(BET)/S_(TEM). The S_(BET) is a specific surface area measured bythe above-mentioned BET method. The S_(TEM) is a value calculated fromthe following equation on the assumption that a particle is arectangular parallelepiped having the average major axial diameter 1 cmand the average minor axial diameter w cm which were measured from theparticles in an electron microscopic photograph:

S _(TEM)(m ² /g)=[(41 w+2 w ²)/1 w ^(2·ρ) _(p)]×10⁻⁴

where ρ_(p) is the density of the hematite particle and 5.2 g/cm² wasused.

The contents of Al, SiO₂ and P were measured by the use of thefluorescent X-ray analysis apparatus 3063 M-type manufactured by RigakuDenki Kogyo Co., Ltd., according to the rules of fluorescent X-rayanalysis of JIS K 0119.

The resin desorption ratio shows a desorbable degree of a resin absorbedonto the acicular hematite particles. As the resin desorption ratio (%)measured by the following method becomes close the zero, the resinbecomes difficult to be desorbed from the surfaces of the acicularhematite particles:

First, 10 g of the acicular hematite particles, 20.5 g of a resinsolution obtained by dissolving 0.5 g of the resin in 20 g of a mixedsolvent (methyl ethyl ketone/toluene/cyclohexanone=5/3/2), and 100 g of1 mmφ glass beads were charged into a 140 ml glass bottle and thecontents were mixed and dispersed for 2 hours by the use of a paintshaker.

Next, the paint composition obtained was taken and introduced into asettling tube and centrifuged at 10000 rpm for 15 minutes to separate asupernatant liquid from a solid. The amount of the resin contained inthe supernatant liquid is weighed and the amount of the resin adsorbedonto the particles Wa (mg/g) is calculated by deducting the measuredamount from the amount of the resin charged.

Next, the solid obtained by the centrifugation is evaporated to drynessand the dried solid containing 5 g of the particles is charged into a140 ml glass bottle. 10 g of the avove-mentioned mixed solvent and 50 gmmφ glass beads are added, mixed and dispersed for 2 hours by the usedof the paint shaker.

The obtained contents are introduced into the settling tube andcentrifuged at 10000 rpm for 15 minutes to separate a supernatant liquidfrom a solid. The amount of the resin desorbed in the supernatant liquidWe (mg/g) is weighed and the resin dersorption ratio is calculated fromthe following equation:

Resin desorption ratio (%)=(We/Wa)×100

The viscosity of the coating composition was measured at 25° C. by theuse of an E type Viscometer EMD-R manufactured by Tokyo Keiki Co., Ltd.,at a shear rate of D=1.92 sec⁻¹.

The gloss was measured at an angle of incidence of 45° by the use of“Glossmeter UGV-5D manufactured by Suga Shikenki, Co., Ltd.

The dispersion stability of the non-magnetic coating composition isrepresented by a change in gloss (%) of the coated film measured by thefollowing method. The smaller the change in gloss, the more excellentthe dispersion stability.

First, the coated film was formed using a non-magnetic coatingcomposition immediately after being dispesed and the angle of incidenceof 45° gloss (Go) is measured. After the non-magnetic coatingcomposition is then left to stand for 60 minutes, the coated film isformed in the samd manner, the angle of incidence of 45° gloss (G) ismeasured, and the change in gloss is measured by the following equation:

Change in gloss (%)=[(G ₀ −G)/G ₀]×100

The surface roughness Ra is represented by an average value of thecenter-line average roughness of the coated film measured by the use of“Surfcom-575A” manufactured by Tokyo Seimitsu Co., Ltd.

The stiffness of the coated film is obtained by measuring the Young'smodulus of the coated film by the use of “Autograph” manufactured byShimadzu Corp. The Young's modulus is represented by a relative valuewith that of a commercially available video tape “AV T-120” manufacturedby Victor Company of Japan, Limited. The higher the relative value, morefavorable.

The magnetic properties were measured under an external magnetic fieldof 10 KOe by the use of “Vibration Sample Magnetometer VSM-15”manufactured by Toei Kogyo Co., Ltd.

The electromagnetic performance of the magnetic tape was obtained by themagnetic tape using the coating composition prepared by the prescriptionas will be described later, which was cut to a ½ inch width, by the useof “Drumtester-BX-3168” manufactured by BELDEX Co., Ltd.

In the case of the magnetic tape using the acicular magnetic iron oxideparticles as the magnetic particles, the electromagnetic performance wasrepresented by a relative value of the output performance at a relativespeed at 5.8 m/s between the magnetic tape and a head and the recordingfrequency of 4 MHz with that of each of the reference tape ofcomparative examples as will be described later.

In the case of the magnetic tape using the acicular magnetic particlesmainly containing iron or the plate-like magnetoplumbite ferriteparticles as the magnetic particles, the electromagnetic performance wasrepresented by a relative value of the output performance at a relativespeed of 3.8 m/s between the magnetic tape and a head and the recordingfrequency of 7 MHz with that of each of the reference tapes ofcomparative examples as will be described later.

The storage stability of the magnetic tape was represented by a changewidth (drop width) in electromagnetic performance measured before andafter the storage in the same manner, in which the electromagneticperformance after the storage was measured after stored at a temperatureof 60° C. and a relative humidity of 90% for 14 days.

The stain on a head after running of the magnetic tape was made byfour-rank evaluation according to the following criteria of the visualobservation of a stain on the head after the magnetic tape was caused torun at a relative speed of 16 m/sec with a load of 200 g for 30 minutesby the use of “Mediadurabilitytester MDT-3000” manufactured by SteinbergAssociates Co., Ltd:

1: No stain is observed.

2: Stain is slightly observed.

3: Stain is observed.

4: Stain is noticeably observed.

The thickness of the non-magnetic base film, the non-magnetic undercoatlayer and the magnetic recording layer forming the magnetic recordingmedium were measured in the following method:

That is, the film thickness (A) of a non-magnetic base film is firstmeasured by the use of a digital electromicrometer “K 351 C”manufactured by Anritsu Electric Co., Ltd. Next, film thickness (B) of anon-magnetic substrate obtained by forming a non-magnetic undercoatlayer on the non-magnetic base film (total of the thickness of thenon-magnetic base film and the thickness of the non-magnetic undercoatlayer) is measured in the same manner. Moreover, the film thickness (C)of the magnetic recording medium obtained by forming a magneticrecording layer on the non-magnetic undercoat layer (total of thethickness of the non-magnetic base film, the thickness of thenon-magnetic undercoat layer and the thickness of the magnetic recordinglayer) is measured in the same manner.

Accordingly, the film thickness of the non-magnetic undercoat layer iscalculated from B minus A and the film thickness of the magneticrecording layer is calculated from C minus B.

<Production of acicular goethite particles and low-density acicularhematite particles>

1500 g of acicular goethite particles obtained by using an aqueousferrous sulfate solution and an aqueous sodium carbonate solution(average major axial diameter: 0.213 μm, average minor axial diameter:0.0246 μm, aspect ratio: 8.7, BET specific surface area(S_(BET)): 113.6m²/g, degree of densification S_(BET)/S_(TEM): 3.43, geometricalstandard deviation in the major axial diameter: 1.36, total content ofsodium calculated as Na: 1864 ppm, soluble sodium salt calculated as Na:301 ppm) were suspended in water to form a slurry, and the solidconcentration was adjusted to 10 g/liter. 150 liters of the obtainedslurry was heated to 60° C. and the pH value was adjusted to 10.0 byaddition of a 0.1 mol/liter aqueous KOH solution.

To the alkali slurry, 30 g of phosphoric acid as a sintering-preventingagent was gradually added and aged for 30 minutes after the terminationof addition. Then, a 0.5 mol/liter aqueous acetic acid solution wasadded to thus adjust the pH value to 6.0. Thereafter, the slurry wasfiltered, washed with water, dried and pulverized by an ordinary methodto thereby obtain acicular goethite particles coated with the phosphoruscompound. The content of the phosphorus compound calculated as P in theacicular goethite particles was 0.63% by weight based on the weight ofthe acicular goethite particles.

1300 g of the acicular goethite particles obtained were charged into astainless steel rotary furnace and dehydrated by heating in air at 320°C. for 30 minutes while rotating the furnace to thereby obtainlow-density acicular hematite particles.

The low-density acicular hematite particles obtained had an averagemajor axial diameter of 0.171 μm, an average minor axial diameter of0.0221 μm, an aspect ratio of 7.7, a BET specific surface area (S_(BET))of 141.6 m²/g, at S_(BET)/S_(TEM) of 3.82, and a geometrical standarddeviation in the major axial diameter of 1.36. The total content ofsodium (calculated as Na) was 1871 ppm, the content of the solublesodium salts (calculated as Na) was 568 ppm and the content of thephosphorus compound (calculated as P) was 0.69% by weight.

<Production of Low-density Acicular Magnetite Particles>

1100 g of the obtained low-density acicular hematite particles werecharged into the stainless steel rotary furnace and reduced by heatingin a hydrogen gas atmosphere at 450° C. for 120 minutes while rotatingthe furnace to thereby obtain low-density acicular magnetite particles.

The low-density acicular magnetite particles obtained had an averagemajor axial diameter of 0.166 μm, an average minor axial diameter of0.0232 μm, an aspect ratio of 7.2, a BET specific surface area (S_(BET))of 54.6 m²/g, an S_(BET)/S_(TEM) of 1.53, and a geometrical standarddeviation in the major axial diameter of 1.37. The total content ofsodium (calculated as Na) was 1896 ppm, the content of the solublesodium salts (calculated as Na) was 1810 ppm and the content of thephosphorus compound (calculated as P) was 0.70% by weight.

<Washing of Low-density Acicular Magnetite Particles With Water>

After 1000 g of the low-density acicular magnetite particles obtainedwere roughly pulverized by the use of a Nara pulverizer, they were addedinto 10 liters of pure water and were encountered for 60 minutes by ahomomixer manufactured by Tokushu Kika Kogyo Co., Ltd.

The obtained slurry of the low-density acicular magnetite particles wasthen mixed and dispersed for one hour at an axial rotation of 2000 rpmwhile being circulated by a horizontal SGM (Dispamat SL manufactured byS.C. Adichem, Co., Ltd. The low-density acicular magnetite particles inthe slurry remaining a 325 mesh (sieve opening:44 μm) was zero %. Theslurry was washed with water by a decantation method.

The washed slurry containing the low-density acicular magnetiteparticles was filtered by the use of a filter press and washed bypassing pure water till the electric conductivity of a filtrate beingnot more than 5 μS, then dried by an ordinary method, and pulverized tothereby obtain low-density acicular magnetite particles.

The low-density acicular magnetite particles obtained had an averagemajor axial diameter of 0.163 μm, an average minor axial diameter of0.0233 μm, an aspect ratio of 7.0, a BET specific surface area (S_(BET))of 53.2 m²/g, an S_(BET)/S_(TEM) of 1.50, and a geometrical standarddeviation the major axial diameter of 1.36. The total content of sodium(calculated as Na) was 140 ppm, the content of the soluble sodium salt(calculated as Na) was 52 ppm and the content of the phosphorus compound(calculated as P) was 0.70% by weight.

<Production of High-density Acicular Hematite Particles>

Next, 800 g of the washed low-density acicular magnetite particles werecharged into a ceramic rotary furnace and oxidized by heating in the airat 73° C. for 30 minutes while rotating the furnace to thereby obtainhigh-density acicular hematite particles.

The high-density acicular hematite particles obtained had an averagemajor axial diameter of 0.159 μm, an average minor axial diameter of0.0235 μm, an aspect ratio of 6.8, a BET specific surface area (S_(BET))of 50.0 m²/g, an S_(BET)/S_(TEM) of 1.42, and a geometrical standarddeviation in the major axial diameter of 1.37. The total content ofsodium (calculated as Na) was 138 ppm, the content of the soluble sodiumsalts (calculated as Na) was 126 ppm and the content of the phosphoruscompound (calculated as P) was 0.70% by weight.

<Washing of High-density Acicular Hematite Particles With Water>

After 800 g of the high-density acicular hematite particles obtainedwere roughly pulverized by the use of a Nara pulverizer, they were addedinto 8 liters of pure water and were encountered for 60 minutes by ahomomixer manufactured by Tokushu Kika Kogyo Co., Ltd.

The obtained slurry of the high-density acicular hematite particles wasthen mixed and dispersed for one hour at an axial rotation of 2000 rpmwhile being circulated by a horizontal SGM (Dispamat SL manufactured byS. C. Adichem, Co., Ltd. The high-density acicular hematite particles inthe slurry remaining a 325 mesh (sieve opening:44 μm) was zero %. Theslurry was washed with water by a decantation method. To be accurate,the slurry concentration at this point was measured and confirmed to be96 g/liter.

The washed slurry containing the high-density acicular hematiteparticles was filtered by the use of a filter press and washed bypassing pure water till the electric conductivity of a filtrate beingnot more than 5 μS, then dried by an ordinary method, and pulverized tothereby obtain high-density acicular hematite particles.

The high-density acicular hematite particles obtained had an averagemajor axial diameter of 0.158 μm, an average minor axial diameter of0.0228 μm, an aspect ratio of 6.9, a BET specific surface area (S_(BET))of 50.2 m²/g, an S_(BET)/S_(TEM) of the 1.39, a geometrical standarddeviation in the major axial diameter of 1.37, and a resin desorptionratio of 8.6%. The total content of sodium (calculated as Na) was 19ppm, the content of the soluble sodium salts (calculated as Na) was 8ppm, the storage stability under a high temperature and a high relativehumidity (soluble sodium salts calculated as Na) was 9 ppm, and thecontent of the phosphorus compound (calculated as P) was 0.69% byweight.

<Production of a Non-magnetic Undercoat Layer>

The obtained high-density acicular hematite particles, a binder resinand a solvent were mixed and kneaded at a solid concentration of 75% byweight by the use of a plast mill for 30 minutes. Thereafter, a givenamount of the kneaded mixture was taken out and charged into a glassbottle together with glass beads and solvents, then mixed and despersedfor 6 hours by a paint conditioner.

The non-magnetic coating composition obtained was given below:

Acicular hematite particles 100 parts by weight Vinyl chloride-vinylacetate copolymer 10 parts by weight resin containing sodium sulfonategroups Polyurethane resin containing 10 parts by weight sodium sulfonategroups Cyclohexanone 44.6 parts by weight Methy ethyl ketone 111.4 partsby weight Toluene 66.9 parts by weight

The obtained non-magnetic coating composition was applied by anapplicator to a 14 μm-thick polyethylene terephthalate film to athickness of 55 μm and dried to thus form a non-mognetic undercoatlayer.

The thickness of the non-magnetic undercoat layer was 3.5 μm.

The gloss of the non-magnetic undercoat layer was 216%, the surfaceroughness Ra was 5.6 nm and the Yung's modulus was 135.

Moreover, after the non-magnet coating composition was left to stand for60 minutes, the undercoat layer was formed on a 14 μm-thick polyethylenterephthalate film in the same manner as above. The gloss of thenon-magnet undercoat layer was 213% and the change in gloss showing thedispersibility of the non-magnetic coating composition was 1.4%.

<Production of Magnetic Recording Medium>

Acicular metal magnetic particles mainly containing iron (average majoraxial diameter:0.103 μm, average minor axial diameter:0.0152 μm, aspectratio:6.8, coercive force:1910 0e, saturation magnetization:136 emu/g),a binder resin and a solvent were mixed and kneaded at a solidconcentration of 78% by weight by the use of a plast mill for 30minutes. The kneaded mixture was charged into a glass bottle togetherwith glass beads and solvents, then mixed and dispersed for 6 hours by apaint conditioner.

Thereafter, a polishing agent, a lubricant and a hardener were furtheradded and the mixture was further mixed and dispersed for 15 minutes.The composition of the magnetic coating composition obtained was givenbelow:

Acicular metal magnetic particles 100 parts by weight mainly containingiron Vinyl chloride-vinyl acetate copolymer 10 parts by weight resincontaining sodium sulfate groups Polyurethane resin containing 10 partsby weight sodium sulfate groups Polishing agent (AKP-30 manufactured 10parts by weight by Sumitomo Chemical Corp) Carbonblack (#3250 Bmanufactured by 1 part by weight Mitsubishi Chemical Corp.) Lubricant(myristic acid:butyl stearate = 1:2) 3 parts by weight Hardener(polyisocyanate) 5 parts by weight Cyclohexanone 64.9 parts by weightMethyl ethyl ketone 162.2 parts by weight Toluene 97.3 parts by weight

The magnetic coating composition obtained was applied by an applicatorto the non-magnetic undercoat layer to a thickness of 15 μm and orientedand dried in a magnetic field, then calendered. The magnetic recordingmedium was subjected to a curing reaction at 60° C. for 24 hours and themultilayered film was slit into a width of 0.5 inch to thereby obtain amagnetic tape.

The thickness of the magnetic recording layer was 1.0 μm.

The coercive force Hc of the obtained magnetic tape was 1980 0e, thesquareness (Br/Bm) was 0.87, the gloss was 236%, the surface roughnessRa was 5.7 nm and the Young's modulus was 137. The electromagneticperformance at a relative speed between the magnetic tape and a head of3.8 m/s and a recording frequency of 7 MHz was +1.1 dB when the magnetictape obtained by Comparative Example 34 as will be described later wasused as the reference tape. The drop width of the electromagneticperformance at a recording frequency of 7 MHz after stored at atemperature of 60° C. and a relative humidity of 70% for 14 days was 0.1dB.

The stain on the head after running of the magnetic tape for 30 minuteswas 1.

Hereinafter, the present invention will be explained in more detail byway of examples and comparative examples, which in no way limit thescope of the present invention.

<Kind of Acicular Goethite Particles>

As the starting material for the production of acicular hematiteparticles, the goethite particles described in the above-mentionedembodiment and acicular goethite particles 1 to 3 set forth in Table 1were prepared.

TABLE 1 Characteristics of acicular geothite particles Content of Totalsoluble Average Average content of sodium major minor Geometrical sodiumsalts axial axial standard (calculated (calculated Kind of aciculardiameter diameter Aspect ratio deviation S_(BET) S_(TEM) S_(BET)/S_(TEM)as Na) as Na) goethite particles (μm) (μm) (—) (—) (m²/g) (m²/g) (—)(ppm) (ppm) Particles described in 0.213 0.0246 8.7 1.36 113.6 33.1 3.431,864 301 the embodiment Geothite particles 1 0.153 0.0188 8.1 1.33171.1 43.4 3.94 1,538 416 #2 0.186 0.0201 9.3 1.35 152.6 40.3 3.78 1,965513 #3 0.265 0.0278 9.5 1.32  83.2 29.1 2.86 2,562 458

<Production of Low-density Acicular Hematite Particles>

Particles to be treated (Precursors) 1 to 4,

COMPARATIVE EXAMPLE 1

Acicular low-density hematite particles to be treated were obtained inthe same manner as in the above-mentioned embodiment except that thekind of acicular goethite particles, the kind of sintering-preventingagents and amounts thereof added, heating and dehydration temperaturesand times were veried.

The main production conditions and the characteristics are shown inTable 2 and Table 3.

TABLE 2 Sintering-preventing treatment A- Heating and Kind of mountdehydration Precursor acicular Cal- added treatment and goethite cu-(wt. Temp. Time Comp. Ex. particles Kind lated %) (° C.) (min) PrecursorParticles Phosphor- P 1.51 320 30 1 described ic acid in the embodimentPrecursor Goethite Colloidal SiO₂ 2.03 340 30 2 particles 1 silicaPrecursor Goethite Phosphor- P 1.01 310 30 3 particles 2 ic acidPrecursor Goethite Colloidal SiO₂ 3.06 370 30 4 particles 3 silica Comp.Particles Colloidal SiO₂ 1.03 340 30 Ex. 1 described silica in theembodiment

TABLE 3 Characteristics of low-density acicular hematite particlesAverage Average Amount Content major minor Geometrical of sintering-Total of soluble Desorption Precursor axial axial standard Aspectpreventing agent content of sodium ratio of and diameter diameterdeviation ratio S_(BET) S_(TEM) S_(BET)/S_(TEM) Calcu- Content sodiumsalts resin Comp. Ex. (μm) (μm) (—) (—) (m²g) (m²/g) (—) lated (wt. %)(ppm) (ppm) (%) Precursor 1 0.173 0.0229 1.36 7.6 146.8 35.8 4.10 P 1.641,888 613 68.3 #2 0.148 0.0185 1.33 8.0 192.1 44.2 4.35 SiO₂ 2.24 1,546712 71.2 #3 0.175 0.0196 1.35 8.9 168.3 41.4 4.06 P 1.11 1,983 583 68.6#4 0.246 0.0268 1.33 9.2 110.0 30.3 3.63 SiO₂ 3.34 2,580 512 69.2 Comp.Ex. 1 0.173 0.0230 1.36 7.5 132.5 35.7 3.71 SiO₂ 1.13 1,872 588 69.3

<Production of Low-density Acicular Magnetite Particles>

Particles to be treated (Precursors) 5 to 8,

COMPARATIVE EXAMPLES 2 TO 4

Acicular low-density magnetite particles were obtained in the samemanner as in the above-mentioned embodiment except that the kind ofparticles to be treated, heating and reduction temperatures and timeswere varied.

The main production conditions and the characteristics are shown inTable 4 and Table 5.

TABLE 4 Precursor Kind of Heating and reduction and particles Temp. TimeComp. Ex. to be treated Atmosphere (° C.) (min) Precursor 5 Precursor 1Hydrogen gas 480 120 Precursor 6 Precursor 2 ″ 400 180 Precursor 7Precursor 3 ″ 410 160 Precursor 8 Precursor 4 ″ 500 100 Comp. Ex. 2Comp. Ex. 1 ″ 450 120 Comp. Ex. 3 Comp. Ex. 1 ″ 650 120 Comp. Ex. 4Comp. Ex. 1 ″ 200 120

TABLE 5 Characteristics of low-density acicular hematite particlesAverage Average Amount Content major minor Geometrical of sintering-Total of soluble Desorption Precursor axial axial standard Aspectpreventing agent content of sodium ratio of and diameter diameterdeviation ratio S_(BET) S_(TEM) S_(BET)/S_(TEM) Calcu- Content sodiumsalts resin Comp. Ex. (μm) (μm) (—) (—) (m²g) (m²/g) (—) lated (wt. %)(ppm) (ppm) (%) Precursor 5 0.168 0.0230 1.36 7.3 53.8 35.7 1.51 P 1.671,879 1,792 78.8 #6 0.142 0.0187 1.34 7.6 63.6 43.8 1.45 SiO₂ 2.28 1,5511,474 75.6 #7 0.170 0.0195 1.35 8.7 52.6 41.7 1.26 P 1.15 1,988 1,92772.1 #8 0.241 0.0266 1.33 9.1 43.8 30.5 1.44 SiO₂ 3.37 2,590 2,492 76.1Comp. Ex. 2 0.170 0.0228 1.37 7.5 51.6 36.0 1.43 SiO₂ 1.15 1,880 1,82173.2 #3 0.140 0.0503 1.83 2.8 27.6 18.0 1.53 SiO₂ 1.15 1,893 1,850 78.6#4 0.170 0.0230 1.36 7.4 116.5  35.7 3.26 SiO₂ 1.14 1,886   712 88.9

<Washing of Low-density Acicular Magnetite Particles With Water>

Particles to be treated (Precursors) 9 to 12

COMPARATIVE EXAMPLES 5 TO 7

Acicular low-density magnetite particles were obtained in the samemanner as in the above-mentioned embodiment except that the kind ofparticles to be treated and presence or absence of a wet-pulverizationwere veried.

The main production conditions and the characteristics are shown inTable 6 and Table 7.

TABLE 6 Wet-pulverization Precursor Kind of Remaining amount andparticles Presence on sieve Comp. Ex. to be treated or absence (wt. %)Precursor 9 Precursor 5 Presence 0 Precursor 10 Precursor 6 Presence 0Precursor 11 Precursor 7 Presence 0 Precursor 12 Precursor 8 Presence 0Comp. Ex. 5 Comp. Ex. 2 Presence 0 Comp. Ex. 6 Comp. Ex. 3 Presence 0Comp. Ex. 7 Comp. Ex. 4 Presence 0

TABLE 7 Characteristics of low-density acicular magnetite particlesafter washing with water Average Average Amount Content major minorGeometrical of sintering- Total of soluble Desorption Precursor axialaxial standard Aspect preventing agent content of sodium ratio of anddiameter diameter deviation ratio S_(BET) S_(TEM) S_(BET)/S_(TEM) Calcu-Content sodium salts resin Comp. Ex. (μm) (μm) (—) (—) (m²g) (m²/g) (—)lated (wt. %) (ppm) (ppm) (%) Precursor 9 0.168 0.0230 1.36 7.3 52.935.7 1.48 P 1.67 128 43 38.6 #10 0.143 0.0186 1.34 7.7 63.9 44.0 1.45SiO₂ 2.27 139 65 41.2 #11 0.170 0.0194 1.35 8.8 53.1 41.9 1.27 P 1.16 99 40 32.6 #12 0.241 0.0265 1.33 9.1 43.2 30.6 1.41 SiO₂ 3.38 180 6336.8 Comp. Ex. 5 0.170 0.0228 1.36 7.5 51.6 36.0 1.43 SiO₂ 1.16 120 6340.3 #6 0.139 0.0504 1.85 2.8 27.3 18.0 1.51 SiO₂ 1.15  98 55 45.6 #70.169 0.0229 1.36 7.4 117.1  35.9 3.26 SiO₂ 1.15 1,287   113  76.9

<Production of High-density Acicular Hematite Particles>

Particles to be treated (Precursors) 13 to 16,

COMPARATIVE EXAMPLES 8 TO 11

Acicular high-density hematite particles were obtained in the samemanner as in the above-mentioned embodiment except that the kind ofparticles to be treated and heating and oxidation temperatures and timeswere veried.

The main production conditions and the characteristics are shown inTable 8 and Table 9.

TABLE 8 Heating and oxidation Precursor Kind of At- and particles mos-Temp. Time Comp. Ex. to be treated phere (° C.) (min) Precursor 13Precursor 9 Air 750 30 Precursor 14 Precursor 10 ″ 700 60 Precursor 15Precursor 11 ″ 730 60 Precursor 16 Precursor 12 ″ 780 30 Comp. Ex. 8Comp. Ex. 5 ″ 750 30 Comp. Ex. 9 Comp. Ex. 5 ″ 880 30 Comp. Ex. 10 Comp.Ex. 5 ″ 500 30 Comp. Ex. 11 Comp. Ex. 4 ″ 700 30

TABLE 9 Characteristics of high density acicular hematite particlesAverage Average Amount Content major minor Geometrical of sintering-Total of soluble Desorption Precursor axial axial standard Aspectpreventing agent content of sodium ratio of and diameter diameterdeviation ratio S_(BET) S_(TEM) S_(BET)/S_(TEM) Calcu- Content sodiumsalts resin Comp. Ex. (μm) (μm) (—) (—) (m²g) (m²/g) (—) lated (wt. %)(ppm) (ppm) (%) Precursor 13 0.163 0.0231 1.37 7.1 50.8 35.7 1.42 P 1.67128 100 51.6 #14 0.139 0.0187 1.35 7.4 60.6 43.9 1.38 SiO₂ 2.28 136 11248.2 #15 0.164 0.0196 1.35 8.4 49.8 41.6 1.20 P 1.16  96  93 41.6 #160.234 0.0268 1.34 8.7 42.6 30.3 1.40 SiO₂ 3.39 182 139 56.8 Comp. Ex. 80.170 0.0229 1.37 7.4 51.8 35.9 1.44 SiO₂ 1.16 122 100 48.9 #9 0.1210.0401 1.86 3.0 31.6 22.4 1.41 SiO₂ 1.15 101  89 39.6 #10 0.169 0.02301.36 7.3 118.0  35.7 3.30 SiO₂ 1.15 121 412 71.6 #11 0.242 0.0269 1.349.0 44.4 30.2 1.47 SiO₂ 3.39 1,883   1,471   82.6

<Washing of High-density Acicular Hematite Particles With Water>

EXAMPLES 1 TO 4 COMPARATIVE EXAMPLE 12

Acicular high-density hematite particles after washing with water wereobtained in the same manner as in the above-mentioned embodiment exceptthat the kind of particles to be treated and presence or absence of awet-pulverization were veried.

The main production conditions and the characteristics are shown inTable 10 and Table 11.

TABLE 10 Wet-pulverization Remaining Example Kind of Presence amount andparticles or on sieve Comp. Ex. to be treated absence (wt. %) Example 1Precursor 13 Presence 0 Example 2 Precursor 14 Presence 0 Example 3Precursor 15 Presence 0 Example 4 Precursor 16 Presence 0 Comp. Ex. 12Comp. Ex. 11 Presence 0

TABLE 11 Characteristics of high-density acicular hematite particlesafter washing with water Content of Average Average Geo- Amount TotalContent of soluble major minor metrical of sinterin- content solublesodium salts Desorption Example axial axial standard Aspect S_(BET)S_(TEM) S_(BET)/ preventing agent of sodium after passage ratio of anddiameter diameter deviation ratio (m²/ (m²/ S_(TEM) Calcu- Contentsodium salts of time *1) resin Comp. Ex. (μm) (μm) (—) (—) g) g) (—)lated (wt. %) (ppm) (ppm) (ppm) (%) Example 1 0.163 0.0230 1.37 7.1 50.335.8 1.40 P 1.67 21  6  7 8.2 #2 0.139 0.0187 1.34 7.4 61.1 43.9 1.39SiO₂ 2.28 32 13 15 7.6 #3 0.163 0.0195 1.35 8.4 48.9 41.8 1.17 P 1.16  8 6  6 6.8 #4 0.234 0.0268 1.34 8.7 42.1 30.3 1.39 SiO₂ 3.39 44 26 2812.3  Comp. Ex. 0.242 0.0270 1.35 9.0 43.6 30.1 1.45 SiO₂ 3.35 365  43165  36.8  12 *1) Soluble sodium salts (calculated as Na) contained inparticles after left to stand at a temperature of 60° C. and a relativehumidity of 90% for 14 days.

<Coating Treatment of High-density Acicular Hematite Particles>

EXAMPLE 5

After 700 g of the high-density acicular hematite particles obtained inExample 1 were roughly pulverized by the use of a Nara pulverizer, theywere added into 7 liters of pure water and were encountered for 60minutes by a homomixer manufactured by Tokushu Kika Kogyo Co., Ltd.

The obtained slurry of the high-density acicular hematite particles wasthen mixed and dispersed for 6 hours at an axial rotation of 2000 rpmwhile being circulated by a horizontal SGM (Dispamat SL manufactured byS.C. Adichem, Co., Ltd. The pH value of the slurry obtained was adjustedto 4.0 by the use of an aqueous 0.1 mol/liter acetic acid solution andpure water was added into this slurry to adjust the slurry concentrationto 96 g/liter. 5 liters of this slurry were heated to 60° C. and 266 mlof an aqueous 1.0 mol/liter aluminum acetate solution (corresponding to1.5% by weight calculated as Al to the high-density acicular hematiteparticles) were added into this slurry and maintained for 30 minutes,then the pH value of the slurry was adjusted to 7.0 by the use of anaqueous 0.1 mol/liter sodium hydroxide solution. The slurry wasmaintained for 30 minutes, then filtered by the use of a filter pressand washed by passing pure water till the electric conductivity of afiltrate being not more than 5 μS in the same manner as in the aboveembodiment, then dried and pulverized to thereby obtain high-densityacicular hematite particles, the surfaces of which were coated withaluminium hydroxide.

The main production conditions and the characteristics are shown inTable 12 and Table 13.

EXAMPLE 6 TO 8

Acicular hematite particles were obtained in the same manner as inExample 5, except that the kind of acicular hematite particles, the kindand the amount of the coating substances were varied.

The main production conditions and the characteristics are shown inTable 12 and Table 13.

TABLE 12 Surface treatment Amount Kind of added Substances coatedacicular (Al or Cal- Amount hematite SiO₂) cu- coated Example particlesKind (wt. %) Kind^(*2)) lated (wt. %) Example 5 Example 1 Aluminium 1.5A Al 1.48 acetate Example 6 Example 2 Colloidal 1.0 S SiO₂ 0.98 silicaExample 7 Example 3 Aluminium 3.0 A Al 2.91 acetate Example 8 Example 4Aluminium 2.0 A Al 1.94 acetate Colloidal 0.5 S SiO₂ 0.48 silica^(*2))A: aluminium hydroxide  S: silicon oxide

TABLE 13 Characteristics of high-density acicular hematite particleswashed with water after surface treatment Content of Average AverageGeo- Amount Total Content of soluble major minor metrical of sinterin-content soluble sodium salts Desorption axial axial standard AspectS_(BET) S_(TEM) S_(BET)/ preventing agent of sodium after passage ratioof diameter diameter deviation ratio (m²/ (m²/ S_(TEM) Calcu- Contentsodium salts of time *1) resin Example (μm) (μm) (—) (—) g) g) (—) lated(wt. %) (ppm) (ppm) (ppm) (%) Example 5 0.163 0.0230 1.37 7.1 51.1 35.81.43 P 1.65 18 3 3 3.4 #6 0.139 0.0188 1.34 7.4 62.1 43.7 1.42 SiO₂ 2.2635 6 6 2.8 #7 0.163 0.0194 1.35 8.4 48.1 42.0 1.14 P 1.13 10 8 8 4.1 #80.234 0.0268 1.34 8.7 41.6 30.3 1.37 SiO₂ 3.32 42 21  21  3.0 *1)Soluble sodium salts (calculated as Na) contained in particles afterleft to stand at a temperature of 60° C. and a relative humidity of 90%for 14 days.

COMPARATIVE EXAMPLE 13

1500 g of acicular goethite particles (average major axialdiameter:0.248 μm, average minor axial diameter:0.0306 μm, aspectratio:8.1, BET specific surface area (S_(BET)):86.5 m²/g, degree ofdensification S_(BET)/S_(TEM):3.24, geometrical standard deviation ofthe major axial diameter;1.53, total alkali content (total amount of Nacalculated and K calculated):113 ppm, soluble sodium salt content (Nacalculated):21 ppm) obtained by using an aqueous ferrous sulfatesolution and an aqueous ammonia solution were suspended in water to forma slurry. To the slurry, 15 g of phosphoric acid were added as asintering-preventing agent, followed by filtration, washing with water,drying and pulverization by an ordinary method to thereby obtainacicular goethite particles with their surfaces coated with thephosphorus compound. The acicular goethite particles contained 0.32% byweight of the phosphorus compound calculated as P.

1300 g of the acicular goethite particles obtained were then chargedinto a ceramic rotary furnace and oxidized by heating in air at 650° C.for 30 minutes while rotating the furnace.

The acicular hematite particles obtained had an average major axialdiameter of 0.236 μm, an average minor axial diameter of 0.0311 μm, anaspect ratio of 7.8, a BET specific surface area (S_(BET)) of 40.8 m²/g,an S_(BET)/S_(TEM) of 1.54, a geometrical standard deviation of themajor axial diameter of 1.37, a total alkali content (total amount of Nacalculated and K calculated) of 102 ppm, a soluble sodium salt content(calculated as Na) of 21 ppm, a storage stability (soluble sodium saltcalculated as Na) under a high temperature and a high humidity of 98 ppmand a content of the phosphorus compound calculated as P of 0.35% byweight.

COMPARATIVE EXAMPLE 14

1500 g of acicular goethite particles (average major axialdiameter:0.236 μm, average minor axial diameter:0.0300 μm, aspectratio:7.9, BET specific surface area (S_(BET)):93.1 m²/g, degree ofdensification S_(BET)/S_(TEM):3.41, geometrical standard deviation ofthe major axial diameter:1.37, total sodium content (calculated asNa):1912 ppm, soluble sodium salt content (calculated as Na):296 ppm)obtained by using an aqueous ferrous sulfate solution and an aqueoussodium carbonate solution were suspended in water to form a slurry. Tothe slurry, 20 g of phosphoric acid were added as a sintering-preventingagent, and the mixture was filtered by an ordinary method and washedwith water by passing deionized water till the electric conductivity ofa filtrate becomes not more than 1 μS, followed by drying andpulverization to thereby obtain acicular goethite particles with theirsurfaces coated with the phosphorus compound. The acicular goethiteparticles contained 0.41% by weight of the phosphorus compoundcalculated as P.

1300 g of the acicular goethite particles obtained were then chargedinto a ceramic rotary furnace and oxidized by heating in air at 700° C.for 30 minutes while rotating the furnace.

The acicular hematite particles obtained had an average major axialdiameter of 0.206 μm, an average minor axial diameter of 0.0309 μm, anaspect ratio of 6.7, a BET specific surface area (S_(BET)) of 41.1 m²/g,an S_(BET)/S_(TEM) of 1.54, a geometrical standard deviation of themajor axial diameter of the major axial diamer of 1.38, a total sodiumcontent (Na calculated) of 1480 ppm, a soluble sodium salt content(calculated as Na) of 116 ppm, a storage stability (soluble sodium saltcalculated as Na) under a high temperature and a high humidity of 388ppm and a content of the phosphorus compound calculated as P of 0.48% byweight.

<Production of a Non-magnetic Undercoat Layer>

EXAMPLES 9 TO 16 COMPARATIVE EXAMPLES 15 TO 24

Non-magnetic undercoat layers were obtained in the same manner as in theabove-mentioned embodiment, except that the kind of the acicularhematite particles was varied.

The main production conditions and the characteristics are shown inTable 14.

TABLE 14 Production of non-magnetic composition Weight CharacteristicsCharacteristics of non-magnetic undercoat layer Kind of ratio of ofnon-magnetic Young's Example acicular particles/ composition Film Changemodulus and hematite resin Viscosity thickness Gloss In gloss Ra(relative Comp. Ex. particles (—) (cP) (μm) (%) (%) (nm) value) Example9 Example 1 5.0 384 3.5 198 1.4 6.0 135 #10 #2 5.0 410 3.5 208 3.2 5.8135 #11 #3 5.0 384 3.4 201 0.9 6.2 135 #12 #4 5.0 307 3.4 198 1.5 6.4138 #13 #5 5.0 333 3.5 204 2.1 6.0 136 #14 #6 5.0 410 3.5 211 3.6 5.6137 #15 #7 5.0 333 3.4 206 1.6 5.8 138 #16 #8 5.0 282 3.5 208 2.0 5.8141 Comp. Ex. Comp. Ex. 5.0 23,040   3.8 121 21.4 36.4 110 15 1 #16 #55.0 3,072   3.6  68 14.6 56.8 93 #17 #6 5.0 768 3.5  32 11.0 71.2 70 #18#7 5.0 20,480   4.1  72 23.9 46.6 81 #19 #8 5.0 640 3.5 173 11.2 14.8111 #20 #9 5.0 410 3.5 101 14.2 18.2 96 #21 #10 5.0 1,024   3.7  86 22.638.2 93 #22 #12 5.0 384 3.5 189 10.1 8.6 125 #23 #13 5.0 287 3.5 16810.6 14.6 121 #24 #14 5.0 205 3.5 172 12.9 13.8 121

<Production of a Magnetic Recording Medium>

EXAMPLES 17 TO 24 COMPARATIVE EXAMPLES 25 TO 37

Magnetic recording media were produced in the same manner as in theabove-mentioned embodiment, except that the kind of the non-magneticundercoat layers and the kind of the magnetic particles were varied.

Meanwhile, the characteristics of the magnetic particles M-1 to M-3 areshown in Table 15.

TABLE 15 Characteristics of magnetic particles BET Average AverageGeometrical specific major axial minor axial standard Aspect surfaceCoercive Saturation Magnetic Kind of diameter diameter deviation ratioarea force magnetization particles magnetic particles (μm) (μm) (—) (—)(m²/g) (Oc) (emu/g) M-1 Metal magnetic particles 0.135 0.0191 1.38 7.153.5 2,240 138.2 M-2 Ba ferrite particles*³⁾ 0.053 0.0160 1.21 3.3 58.22,510 52.6 M-3 Co-coated magnetite 0.180 0.0252 1.35 7.1 41.6   968 78.6particles *³⁾With respect to Ba ferrite particles, the plate diameter,the thickness and the plate ratio (plate diameter/thickness) wereregarded as “average major axial diameter”, “ average minor axialdiameter” and “aspect ratio”, respectively.

The main production conditions and the characteristics are shown inTable 16 and Table 17.

The electromagnetic performance in Examples 17, 18 and ComparativeExamples 25 to 34 were represented by values obtained by using themagnetic tape of Comparative Example 34 as the reference tape.

The electromagnetic performance in Examples 19 and 20 were representedby values obtained by using the magnetic tape of Comparative Example 35as the reference tape.

The electromagnetic performance in Examples 21 and 22 were representedby values obtained by using the magnetic tape of Comparative Example 36as the reference tape.

The electromagnetic performance in Examples 23 and 24 were representedby values obtained by using the magnetic tape of Comparative Example 37as the reference tape.

TABLE 16 Characteristics of magnetic recording medium Production ofmagnetic Film recording medium thickness Kind of Weight of Drop width innon- ratio of magnetic Coer- Young's Electromagnetic electromagnetic Ex-magnetic particles/ recording cive Br/ modulus Stain performanceperformance*⁴⁾ am- undercoat Kind of mag- resin layer force Bm Gloss Ra(Relative on head 4 MHz 7 MHz 4 MHz 7 MHz ple layer netic particles (—)(μm) (Oe) (—) (%) (nm) value) (—) (dB) (dB) (dB) (dB) Ex- Example 9Particles in the 5.0 1.1 1,990 0.87 238 5.7 137 1 — +1.2 — 0.2 am-embodiment ple 17 #18 #10 Particles in the 5.0 1.0 1,996 0.87 241 5.6135 2 — +1.3 — 0.1 embodiment #19 #11 M-1 5.0 1.0 2,310 0.88 235 5.2 1382 — +2.8 — 0.3 #20 #12 M-1 5.0 1.0 2,323 0.89 239 5.3 139 1 — +2.6 — 0.4#21 #13 M-2 5.0 1.1 2,566 0.86 266 4.8 136 1 — +2.1 — 0.3 #22 #14 M-25.0 1.0 2,532 0.86 258 5.0 136 1 — +2.8 — 0.5 #23 #15 M-3 5.0 1.1 1,0420.90 183 5.2 143 1 +2.6 — 0.1 — #24 #16 M-3 5.0 1.0 1,052 0.91 188 5.6140 1 +3.1 — 0.1 — *⁴⁾Drop width in the electromagnetic performance ofthe magnetic tape after stored at a temperature of 60° C. and a relativehumidity of 90% for 14 days.

TABLE 17 Characteristics of magnetic recording medium Production ofmagnetic Film recording medium thickness Kind of Weight of Drop width innon- ratio of magnetic Coer- Young's Electromagnetic electromagneticmagnetic Kind of particles/ recording cive Br/ modulus Stain performanceperformance*⁴⁾ Comp. undercoat magnetic resin layer force Bm Gloss Ra(Relative on head 4 MHz 7 MHz 4 MHz 7 MHz Ex. layer particles (—) (μm)(Oe) (—) (%) (nm) value) (—) (dB) (dB) (dB) (dB) Comp. Comp. Particlesin 5.0 1.3 1,921 0.78 186 13.8 113 3 — −1.5 — 2.6 Ex. Ex. the em- 25 15bodiment #26 #16 Particles in 5.0 1.4 1,930 0.77 132 32.8 98 4 — −3.8 —1.8 the em- bodiment #27 #17 Particles in 5.0 1.1 1,968 0.82 116 41.6 814 — −4.1 — 1.6 the em- bodiment #28 #18 Particles in 5.0 1.3 1,932 0.80121 38.8 93 4 — −3.6 — 1.9 the em- bodiment #29 #19 Particles in 5.0 1.01,978 0.84 204  8.2 115 3 — −0.8 — 2.3 the em- bodiment #30 #20Particles in 5.0 1.1 1,966 0.81 176 16.6 100 4 — −1.8 — 2.0 the em-bodiment #31 #21 Particles in 5.0 1.3 1,960 0.79 156 25.8 96 4 — −2.2 —2.2 the em- bodiment #32 #23 Particles in 5.0 1.1 1,967 0.84 180 11.2124 3 — −1.6 — 1.3 the em- bodiment #33 #24 Particles in 5.0 1.0 1,9700.84 184 11.0 124 3 — −2.0 — 1.2 the em- bodiment #34 #22 Particles in5.0 1.1 1,976 0.84 215  9.6 128 3 — (0.0) — 1.6 the em- bodiment #35 #22M-1 5.0 1.0 2,295 0.83 208 11.2 127 3 — (0.0) — 1.8 #36 #22 M-2 5.0 1.12,537 0.80 199 16.8 123 3 — (0.0) — 1.6 #37 #22 M-3 5.0 1.0 1,021 0.85201 8.8 133 3 (0.0) — 1.2 — *⁴⁾Drop width in the electromagneticperformance of the magnetic tape after stored at a temperature of 60° C.and a relative humidity of 90% for 14 days.

The acicular hematite particles for a non-magnetic undercoat layer of amagnetic recording medium according to the present invention have anexcellent dispersibility in a vehicle so that it is possible to enhancethe surface smoothness and the strength of the non-magnetic undercoatlayer. When a magnetic recording layer is formed on the non-magneticundercoat layer, not only can the magnetic recording layer be formedinto a smooth and uniform thin film, but also a magnetic recordingmedium having an excellent electromagnetic performance and an excellentstorage stability can be obtained.

The magnetic recording medium according to the present invention uses asparticles for a non-magnetic undercoat layer acicular hematite particleswith the total content of sodium of not more than 50 ppm which isexcellent in storage stability, so that it is excellent in storagestability as well as electromagnetic performance.

What is claimed is:
 1. A method for producing particles for anon-magnetic undercoat layer of a magnetic recording medium, whichcomprises the steps of: dehydrating acicular goethite particles with thesurfaces coated with a sintering-preventing agent to form acicularhematite particles, reducing the acicular hematite particles at atemperature of 250 to 600° C. under a reducing atmosphere to formacicular magnetite particles, washing with pure water and drying theacicular magnetite particles, oxidizing the acicular magnetite particlesat a temperature of 650 to 850° C. under an oxidizing atmosphere, andwashing with pure water and drying the resulting high-density acicularhematite particles.
 2. A method for producing particles for anon-magnetic undercoat layer of a magnetic recording medium, whichcomprises the steps of: dehydrating acicular goethite particles to formacicular hematite particles, coating the surfaces of the acicularhematite particles with a sintering-preventing agent, reducing theacicular hematite particles at a temperature of 250 to 600° C. under areducing atmosphere to form acicular magnetite particles, washing withpure water and drying the acicular magnetite particles, oxidizing theacicular magnetite particles at a temperature of 650 to 850° C. under anoxidizing atmosphere, and washing with pure water and drying theresulting high-density acicular hematite particles.
 3. A method of claim1 or 2, wherein the particles are wet-pulverized prior to washing withpure water.
 4. A method of claim 1 or 2, wherein the high-densityacicular hematite particles are coated with at least one selected fromthe group consisting of an aluminium hydroxide, an aluminium oxide, asilicon hydroxide and a silicon oxide by treating the particles with anaqueous solution containing an aluminium compound, a silicon compound orthe both compounds.